Adenoid cystic carcinoma (ACC) of the breast is an uncommon carcinoma accounting for less than 1% of the breast carcinoma. This type of carcinoma has a distinctive histology, such as cylindromatous, cribriform, glandlike space and solid pattern in the variable proportion. Prognosis is favorable with rare recurrence or metastasis and the guideline for therapy is not well established. We experienced a case of ACC of right breast in the 48 year-old woman. Cytologically, it showed numerous three-dimensional, well outlined cell clusters with central core of homogeneous, eosinophilic material. The mass was well demarcated, firm and measured 2.5 2.5 2 cm. Cut surface was yellowish white and granular with focal necrosis. Histologically, the tumor was composed of cylindromatous, adenoid and solid area with marked cellular atypia. Focal invasion was identified in the adipose tissue around the tumor. Histologic and nuclear grade was III by Kleer's grade. Nodal metastasis was not found.